[interfaces-config.sh] Force lo interface down

[jleveque]

- What I did
Force "lo" interface down in interfaces-config.sh to prevent interface-config.service from failing with the following error:

-- The result is failed.
systemd[1]: networking.service: Unit entered failed state.
systemd[1]: networking.service: Failed with result 'exit-code'.
interfaces-config.sh[29232]: Job for networking.service failed because the control process exited with error code.
interfaces-config.sh[29232]: See "systemctl status networking.service" and "journalctl -xe" for details.
interfaces-config.sh[29232]: ifdown: interface lo not configured
interfaces-config.sh[29232]: RTNETLINK answers: File exists
interfaces-config.sh[29232]: ifup: failed to bring up lo
systemd[1]: interfaces-config.service: Main process exited, code=exited, status=1/FAILURE
systemd[1]: Failed to start Update interfaces configuration.
-- Subject: Unit interfaces-config.service has failed

Failure to bring down the interface will result in a failure to subsequently bring the interface back up.

- How I did it
Add --force flag

- How to verify it
Repeatedly test that systemctl restart interfaces-config.service doesn't fail to bring down the lo interface.

[lguohan]

@prsunny , we are using lo for loopback for master branch now? do you really need this ifdown/up lo in the interfaces-config.sh?

[lguohan]

i suggest we remove the whole line for master and 201911 branch. and have this patch only on 201811.

[prsunny]

We don't use this anymore in master or 201911. The loopback interface is Loopback0 instead of lo. So its no-op in master/201911. Agree with Guohan that its relevant only for 201811.

[jleveque]

Closing this PR. I have opened #4149 for he 201811 branch and I will open a new PR to remove this line in master/201911.